The disclosed invention relates to a process for reducing high-frequency energy noise components in the output radiation of a laser, to a process for generating short pulses, and to lasers utilized for short pulses in a particularly simple, rugged, and compact structure.
High-frequency energy noise components of a laser output radiation are understood to mean energy noise components in the frequency range of 100 kHz to several GHz, preferably of 100 kHz to 4 MHz.
The process of this invention and the laser in accordance with this invention result, in a continuous laser, in a reduction of the high-frequency energy noise components in the output radiation and, in case of a pulsed laser, due to the reduction of the noise components, in a stabilization of the peak intensity and the pulse duration of the laser pulses.
The spontaneous radiation emission of a laser is understood to mean a radiation of the same radiation frequency as the laser output radiation which, in case of a laser that is pumped, but not oscillating, extends in the direction in the laser cavity wherein also the laser output radiation would extend in case of an oscillating laser. A pumped laser can be prevented from oscillating, for example, by tilting its resonator mirrors.
Mode-coupled lasers are understood to mean lasers for the generation of pulses in the pico- and sub-picosecond ranges wherein an active or passive loss, amplification or phase modulation is performed in the laser cavity.
A process for generating short laser pulses with the use of a feedback path containing optically nonlinear components has been known from the publication by P. N. Kean et al., "Enhanced Mode Locking of Color-Center Lasers", Opt. Lett. vol. 14, No. 1, Jan. 1, 1989, pp. 39-41. In the conventional method, a color-center laser with a KCl:Tl color-center crystal is continuously pumped with a mode-locked Nd:YAG laser. The reflection of the coupling-out mirror of the laser cavity amounts to 80%. A portion of the laser output power is coupled into a germanium-containing, single-mode waveguide having a length of 2.2 m by means of a beam splitter outside of the cavity on the side of the coupling-out mirror. A mirror is mounted at the end of the waveguide; this mirror reflects the light exiting from the waveguide back into the latter. The distance between the end of the waveguide and the waveguide mirror is adjustable by means of a piezoelectric shifting device (PZT). The energy coupled into the waveguide can be attenuated by means of a neutral density filter (ND). The laser radiation coupled into the waveguide by the beam splitter passes through the waveguide, is reflected on the waveguide mirror, passes a second time through the waveguide, and is again coupled into the laser cavity via the beam splitter and the coupling-out mirror. On the basis of optical nonlinearities in the waveguide, caused by the Kerr effect induced by the laser pulse, a frequency expansion of the pulse propagated in the waveguide takes place. The setting of an optimum phase relationship between the laser radiation in the laser cavity and the radiation fed back via the waveguide is effected by means of an electronic evaluating circuit which moves the waveguide mirror by way of the shifting device into the optimum distance with respect to the waveguide end. By the superposition of the laser pulses oscillating in the laser cavity with the pulses frequency-expanded by the waveguide, laser pulses are produced in the laser cavity having pulse widths in the pico- and sub-picosecond ranges. If additionally the energy coupled into the waveguide is optimized by means of the neutral density filter, extremely short laser pulses are obtained on the order of magnitude of 1.1 ps (picoseconds=10.sup.-12 seconds). With a shortening of the waveguide to a length of 24 cm, laser pulses were obtained having a pulse width of 260 fs (femtoseconds=10.sup.-15 seconds).